This invention relates to the use of polar copolymer/nonpolar polymer blends as thermoplastic outer surface coating layers for paper and paperboard products. The novel coatings function to: maintain acceptable treatment levels over a long period of time; have normal heat-sealing properties; and have good gluing and printing characteristics.
Exterior coatings which have been applied to paper and/or paperboard layers function to provide surfaces for heat-sealing with interior laminate layers during package formation steps. They also function as an exterior moisture barrier. Additionally, these exterior coating layers are surfaces which must be printable and receptive to gluing operations.
Typically, the outer or exterior coating layer for a paper or paperboard substrate has consisted of a nonpolar thermoplastic polyolefin polymer. Common polymers utilized for these coating layers are polymers such as low density polyethylene, linear low density polyethylene, medium density polyethylene, linear medium density polyethylene, high density polyethylene, polypropylene, as well as blends thereof. These commonly used polymers, all nonpolar in nature, are economically desirable and heat-seal well with like nonpolar polymers, which are often utilized either alone or coextruded with barrier polymers for an interior layer of a laminate packaging, or container structure.
These commonly used nonpolar polymers cause problems during carton printing especially when water based polar inks are used. These water based polar inks do not adhere well to the commonly used nonpolar polymer coating layer. Additionally, significant problems occur during gluing, due to the nonpolar nature of the coating layer. These water based materials will "bead-up" on the surface of the nonpolar polymer coatings. As a result, poor coverage and adhesion occurs.
In the past, corona discharge treatment or flame treatment of the coating layer has been employed to improve the wettability of the nonpolar polymer layer. The surface characteristics of these nonpolar polymer coatings can be modified by corona discharge or direct flame treatment to make the surface more polar, and therefore, more wettable by water based inks and glues. This treatment is required for good ink adhesion in printing, and for good glue adhesion for package sealing. Corona discharge treatment levels and, to a lesser degree, direct flame treatment levels, which are directly proportional to wettability, decline proportionally over time. Extrusion coaters frequently treat production to very high levels with the intention that the customer receive an acceptable treatment level after the normal delays of shipping and storage. However, corona discharge treating of nonpolar polymers to very high levels often results in the production of numerous polymer pinholes. These pinholes are formed by burning through the web due to treater arcing. Such pinholes compromise package integrity. Treatment loss is a particular problem on material stored for long periods of time, or shipped overseas, due to the long transit time. Transit time can result in delays of several months to a year from the time of production to the time of printing and converting. Consequently, pretreated coatings must be retreated to raise the dissipated treatment levels. Such pretreatment is time-consuming and expensive.
The use of direct flame treatment produces a more permanent treatment level, but often results in streaks, loss of gloss, or other imperfections in high-gloss polyethylene or polypropylene surfaces.
The need for water-wettable printing surfaces on nonpolar polymers has increased. In the past, printers and converters could use solvent based inks and glues to get satisfactory adhesion to untreated nonpolar polymer surfaces. These solvent based inks and glues cause environmental problems due to solvent emissions into the air resulting from ink and glue drying and waste water stream pollution from press clean up. As a result, most printing and converting operations have changed to water based inks and glues, rather than installing expansive and expensive solvent recovery and clean up systems.
It is an objective of the present invention to economically produce thermoplastic type polymer coated paper or paperboard products that maintain treatment levels over a long period of time.
It is a further object of the present invention to economically produce thermoplastic type polymer coated paper or paperboard products that have normal heat sealing, gluing, and printing characteristics.
Applicant is owner of numerous patents which relate to paper or paperboard coated products such as U.S. Pat. Nos. 4,698,246; 4,701,360; 4,789,575; 4,802,943; 4,806,399; 4,859,513; 4,861,526; 4,880,701; 4,888,222; 4,921,733; and 4,929,476. These laminate structures use polyethylene, polypropylene, and other nonpolar polymers as an exterior coating layer.